The present invention relates to a device for the non-destructive quality-testing of a joint obtained by electrical spot-welding of two sheets joined together in a local zone with the formation of a core.
More particularly the present invention relates to a device for the non-destructive testing of the thickness of the welding core.
At present, known devices for the non-destructive testing of weld quality are based on ultrasonic testing and generally comprise an ultrasound generator and a detector for detecting an ultrasonic return signal. In use, the ultrasound generator is placed near the test piece so that ultrasonic waves pass through the test piece, the returning signal arriving at the detector delayed in proportion to the thickness of material traversed by the signal.
In the specific case of weld testing, if a weld core has not been formed, the ultrasound will be reflected at the air gap between the two pieces. If, on the contrary, there is no air between the two pieces, the detector will receive a signal confirming the presence of a weld.
The above described known devices, even if widely used, are not, however, suitable for measuring weld core thickness. Such devices simply reveal whether or not there is a discontinuity between the test pieces, but do not indicate whether a weld core or a simple bond has been formed. Furthermore the use of these devices is difficult in a factory, since they are bulky and the information which they provide is not sufficiently clear. It is known that whenever radiant heat impinges on the surface of a metallic object the radiation is partly absorbed and partly reflected by the object. The absorbed radiation causes a gradual heating of the whole object, due to the thermal diffusivity of the material.
There are devices able to determine the thermal diffusivity in a homogeneous metallic piece, the measured thermal diffusivity being characteristic of the material, whose composition may be unknown.